1. Field of the Invention
The present invention relates to an electromagnetic clutch of a refrigerant compressor used for an automotive air-conditioning system. More particularly, it relates to a bobbin of an electromagnet for which the shape distortion that may occur after injection molding is reduced and the process of filling resin into the bobbin core may be improved.
2. Description of Related Art
With reference to FIG. 1, an electromagnetic clutch 400 of a refrigerant compressor 500 used for an automotive air-conditioning system generally consists of three subassemblies. These subassemblies are a core assembly 100, a rotor assembly 200, and an armature assembly 300. With reference to FIG. 2, core assembly 100 is made of core 150, coil assembly 10(10'), and resin 160. Coil assembly 10(10') is contained in a circular recess 151 of core 150, and is fixed by resin 160 which fills circular recess 151.
Basically, coil assembly 10 is made of a bobbin and a coil wound around the bobbin. In FIGS. 3, 4, and 5, the shape is shown of a bobbin according to the prior art. With reference to FIGS. 3-5, a bobbin 1 is constructed from a cylinder 2 and flanges 3 and 4, each of which extends in a radially outward direction from an opposite end of cylinder 2. For flanges 3 and 4, detailed structures are provided. With reference to FIG. 3, flange 3 has two ridges 32a and 32b which accommodate a thermal fuse (not shown) therebetween, two protrusions 33a and 33b on which lead wires (not shown) of the thermal fuse are hooked; two shallow recesses 34a and 34b which partially accommodate the pressure connection terminals that connect coil wires (not shown) and lead wires of the thermal fuse; and two notches 35a and 35b which introduce part of the coil wire to the outer surface of the flange 3. With reference to FIG. 5, flange 4 has an excision 43 and a protrusion 42 which has clip portions 42a and 42b for securing a terminal portion of the coil wire.
Bobbin 1 is made of synthetic resin, such as a nylon 6,6 containing glass fiber, and is formed by an injection molding process. With reference to FIG. 3, on the outer surface of flange 3 are three points 36, 36', and 36" that correspond to the gates of a metal mold used for injecting the resin. In an injection molding process of bobbin 1, resin, which is injected through points 36, 36', and 36" under high pressure, is cooled down within the metal mold while streaming and solidifying. The resin fills the metal mold to shape bobbin 1. During such injection molding, the streams of resin injected through points 36, 36', and 36" meet each other approximately at weldlines 37, 37', 37", 38, 38', and 38" and produce weldlines which do not have homogeneous physical properties. Weldlines 37, 37', 37", 38, 38', and 38" occur at locations substantially equidistant from points 36, 36', and 36".
The foregoing method for manufacturing bobbin 1 results in at least two serious defects. First,, distortion stress concentrates and persists at weldlines 37, 37', 37", 38, 38' and 38" and is very difficult to avoid in any injection molding process. Consequently, flanges 3 and 4 warp inward or outward over time after the injection molding is completed, as shown in FIGS. 6 and 7. FIG. 6 depicts bobbin I in which both flanges 3 and 4 have warped outward by the amount +e. Outward warping of flanges 3 and 4 is not necessarily a problem if it is contained within a tolerance limit. Inward warping of flanges 3 and 4, i.e., of amount -e, as depicted in FIG. 7, however, may be a severe problem. Even if the inward warp is slight, the coil wire can not be wound around bobbin 1 by automatic machinery. It is extremely difficult to reduce or eliminate the warping of flanges 3 and 4 in such bobbin structures.
Second, in known bobbin structures, the surface area of flanges 3 and 4 of bobbin 1, are too broad and hinder the laminar flow of resin into circular recess 151 of core 150. With reference to FIG. 2, core assembly 100 is made from core 150 and coil assembly 10 which is situated in circular recess 151 of core 150. A resin 160 is poured to fix the coil assembly 10 in the circular recess 151 of the core 150. The melting temperature of the thermal fuse is about 180.degree. C. In order not to break the thermal fuse while filling circular recess 151 of core 150, the temperature of resin 160 flowing into circular recess 151 must be lower than the thermal fuse melting temperature. Often, an epoxy thermosetting resin is used for resin 160 because its melting temperature is about 60.degree. C. and its thermosetting temperature is about 120.degree. C. This process of filling resin 160 into circular recess 151 of core 150 is called the potting process and depends on a spontaneous streaming of resin 160. The surface area of flanges 3 and 4 of bobbin 1 is substantially the same as the area of circular recess 151, which hinders resin 160 from spreading adequately and may result in bubbles forming within resin 160. Water may readily intrude into resin 160 via these bubbles from the exterior of clutch 400. As a consequence, serious problems occur, such as the breaking down of the electrical insulation between the coil wire and core 150 or a reduction of the resistance of core assembly 10 to the effects of vibration.